Brown tree snake wiped out Guam’s bird species

Washington, Aug 8 (IANS) Brown tree snakes which caused the extinction of nearly every native bird on the Pacific island of Guam could be inflicting even more severe consequences indirectly, according to a new study. The invasive species might have even changed tree distribution and reduced native tree populations, harming the already damaged ecosystem even further, according to Washington University (WU) biologists.

“The brown tree snake has often been used as a textbook example of the negative impacts of invasive species, but after the loss of birds no one has looked at the snake’s indirect effects,” said Haldre Rogers, a WU doctoral student.

“It has been 25 years since the birds disappeared. It seems to me the consequences are going to keep reverberating throughout the community if birds are fundamental components of the forest,” she said.

Birds typically make up a small part of the life of a forest, but they are important for pollination, spreading seeds around the forest and controlling insects that feed on plants.

Guam, an island nearly 50 km long and 8-20 km wide, lost most of its native birds after the brown tree snake was introduced by accident from the Admiralty Islands following World War II.

The snake has few predators on Guam, so its population density is quite high and some individuals grow to the unusual length of 3 metres.

Before introduction of the brown tree snake, Guam had 12 species of native forest birds. Today 10 of those are extinct on Guam, and the other two species have fewer than 200 individuals.

Though Guam has some non-native bird populations, few other birds moved in when native species died out, and none of them live in the forest. That leaves few birds to consume tree seeds and then drop them away from the trees.

That could have two possible negative impacts on the native forests, Rogers said. First, some plant species need birds to handle their seeds to ensure effective germination. In addition, seed predators and fungi that kill seeds are often found in high density directly beneath a parent tree, so the trees rely on birds to disperse seeds beyond the range of those negative effects.

If native birds performed those functions on Guam, tree populations could suffer from the loss of birds. It appears 60-70 percent of tree species in the native forests are dispersed, at least in part, by birds, she said.

“These findings could have global implications, since forests in areas that have had a decline in bird populations instead of outright extinction might show effects similar to those in the forests of Guam,” Rogers said.

Rogers presents her data Friday at the Ecological Society of America meeting in Milwaukee. Her co-authors are Joshua Tewksbury and Janneke Hille Ris Lambers, both WU assistant professors of biology.